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Homework Help: Integration by Parts query

  1. Jun 17, 2011 #1
    Basically I have answered a question using the integration by parts formulae to work out the centre of gravity inside a fan blade using :-

    v.du/dx = v.u - u. dv/dx with the integral limits of 0 ==> 20

    when v = x then dv/dx =1
    when du/dx = 0.3 sinx then u = 0.3cos x

    and sub this into the equation to get x(0.3sinx) = x(0.3cosx) - 0.3cosx(1)

    Can anyone see any issue with this because he's marked it wrong?
  2. jcsd
  3. Jun 17, 2011 #2


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    Science Advisor

    Well, one problem is that if du/dx= 0.3 sin x then u= -03 cos x, not 0.3 cos x

    And you say the integral is from 0 to 20. Did you evaluate between those two limits? The correct answer is a number, not a formula in x.
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